add test for loader-provided source

test/es-module/test-esm-loader-with-source.mjs

@@ -0,0 +1,10 @@
+// Flags: --experimental-loader ./test/fixtures/es-module-loaders/preset-cjs-source.mjs
+import '../common/index.mjs';
+import * as fixtures from '../common/fixtures.mjs';
+import assert from 'assert';
+
+const { default: existingFileSource } = await import(fixtures.fileURL('es-modules', 'cjs-file.cjs'));
+const { default: noSuchFileSource } = await import(fixtures.fileURL('no-such-file.cjs'));
+
+assert.strictEqual(existingFileSource, 'no .cjs file was read to get this source');
+assert.strictEqual(noSuchFileSource, 'no .cjs file was read to get this source');

test/fixtures/es-module-loaders/preset-cjs-source.mjs

@@ -0,0 +1,21 @@
+export function resolve(specifier, context, next) {
+  if (specifier.endsWith('/no-such-file.cjs')) {
+    // Shortcut to avoid ERR_MODULE_NOT_FOUND for non-existing file, but keep the url for the load hook.
+    return {
+      shortCircuit: true,
+      url: specifier,
+    };
+  }
+  return next(specifier);
+}
+
+export function load(href, context, next) {
+  if (href.endsWith('.cjs')) {
+    return {
+      format: 'commonjs',
+      shortCircuit: true,
+      source: 'module.exports = "no .cjs file was read to get this source";',
+    };
+  }
+  return next(href);
+}